The invention relates to mass spectrometers, in particular to mass spectrometers having a plurality of analyzers and including at least one magnetic sector.
A mass spectrometer is an instrument for analyzing a sample by ionizing at least some of the sample and analyzing the ions formed according to their mass-to-charge ratios. Many different types of analyzers are used in mass spectrometers. These include the magnetic sector analyzer, in which ions are subjected co a magnetic field which disperses the ions according to their mass-to-charge ratio. Other analyzers include the quadrupole analyzer, in which a varying quadrupole field is used to selectively transmit only ions with a particular mass-to-charge ratio, and the time-of-flight analyzer which analyses ions according to their velocities. Many other types and subdivisions of types also exist such as the ion cyclotron resonance analyzer, the ion trap analyzer, the Wien Filter etc.
A typical mass spectrometer may contain one or more analyzers of the same or different types which are combined together in a way which optimizes the parameters of the instrument depending on its intended use. For example, in a double-focusing mass spectrometer, magnetic and electrostatic analyzers are combined to effect direction and velocity focusing (e.g. see Chapter 5 of "Mass Spectroscopy" (2nd ed.), Duckworth et al, CUP 1990). Double-focusing mass spectrometers, and electrostatic analyzers suitable for use in such mass spectrometers, are also disclosed in U.S. Pat. No. 5,194,732 and U.S. Pat. No. 5,198,666. Mass spectrometers are also known in which magnetic sectors are combined with quadrupole analyzers or with time-of-flight (TOF) analyzers.
A typical prior mass spectrometer having both a magnetic sector and a TOF analyzer is shown in FIG. 1 The mass spectrometer 1 comprises an ion source 2 which may be of any of a selection of conventional types, e.g. electron impact, chemical ionization, Electrospray or Field Desorption etc. A sample introduced into the ion source is ionized and a beam 3 of ions is formed which passes from the source through a source slit 4, an alpha-angle defining slit 5, a first electrostatic analyzer 6, a magnetic sector analyzer 7, and a second electrostatic analyzer 8. The combination of the velocity focusing due to the first and second electrostatic analyzers and the momentum focusing due to the magnetic sector gives rise to a double-focused mass-dispersed ion image at a collector slit 9. An off-axis ion detector 10 is disposed downstream of the collector slit 9 and produces an electrical signal indicative of the number of ions in a given range of mass-to-charge ratios which pass through the collector slit.
A time-of-flight (TOF) analyzer 11 is also disposed downstream of the collector slit 9. Only ions whose mass-to-charge ratios fall within a range determined by the width of the collector slit 9 pass into the TOF analyzer 11 at any one instant. A collision cell 13 containing an inert gas at a relatively high pressure is disposed in the path of the ion beam 12 between the collector slit 9 and the TOF analyser 11 to cause controlled fragmentation of the ions passing through the collector slit 9. Structural information may then be obtained by TOF mass analysis of the daughter ions so formed. Since daughter ions formed by high energy collisions all have the same velocity but typically have different axial energies, an orthogonal-acceleration TOF mass analyzer is well-suited for the analysis of these collision products. A second off-axis ion detector 14 is disposed downstream of the TOF analyser 11.
Operation of the orthogonal-acceleration TOF analyzer portion of the mass spectrometer shown in FIG. 1 is as follows. Ions 12 of a selected mass-to-charge ratio enter the collision cell 13 and are fragmented by collisions with molecules of the inert gas. Daughter ions so formed pass through a deceleration region 15 and an extraction region 16. The potential of a repeller electrode 17 is pulsed in such a way that a packet of ions is repelled from it and travels towards a third ion detector 18. Measurement of the time interval between the electrical pulse applied to the repeller electrode 17 and the arrival of the packet of ions received at the detector 18 allows the mass-to-charge ratio of the daughter ions to be determined.
Such an instrument is described by R. H. Bateman, M. R. Green and G. Scott in Proc. 42nd ASMS Conf. Mass Spectrom. 1994, p 1034.
The prior instrument of FIG. 1 offers many advantages over a magnetic sector instrument. For example, the use of a magnetic sector offers unit mass ion selection with high transmission whereas the TOF analyzer offers the potential for high sensitivity and the acquisition of a full product ion spectrum. However, the TOF analyzer 11 cannot be used to mass analyze the ion beam 19 from the magnetic sector spectrometer because only ions of a limited range of mass-to-charge ratios can be transmitted through the collector slit 9 to the TOF analyzer 11 at ally instant. This limitation applies to any spectrometer comprising a magnetic sector in combination with one or more other analyzers, in that the magnet will always introduce mass dispersion so that only selected ions pass into the next stage.